docs: def true_num_genes(model, custom_spont_id=None):
      """Return the number of genes in a model ignoring spontaneously labeled genes.
  
      Args:
          model (Model):
          custom_spont_id (str): Optional custom spontaneous ID if it does not match the regular expression ``[Ss](_|)0001``
  
      Returns:
          int: Number of genes excluding spontaneous genes
  
      """
      true_num = 0
      for gene in model.genes:
          if not is_spontaneous(gene, custom_id=custom_spont_id):
              true_num += 1
      return true_num